Learning Outcomes
Describe a sample an are and straightforward and compound occasions in it using traditional notationCalculate the probability of an occasion using standard notationCalculate the probability of 2 independent events using conventional notationRecognize when two events are mutually exclusiveCalculate a conditional probability using standard notationProbability is the likelihood the a specific outcome or occasion happening. Statisticians and also actuaries usage probability to make predictions around events. One actuary that works for a vehicle insurance firm would, for example, it is in interested in just how likely a 17 year old male would be to obtain in a auto accident. They would usage data from past occasions to do predictions around future occasions using the characteristics of probabilities, then usage this details to calculation an insurance money rate.
You are watching: Is the likelihood that a given event will occur.
In this section, we will discover the an interpretation of an event, and also learn how to calculate the probability that it’s occurance. We will likewise practice using typical mathematical notation come calculate and describe different kinds the probabilities.
Basic Concepts
If you role a die, choose a card from deck of play cards, or randomly choose a person and observe your hair color, we are executing an experiment or procedure. In probability, we look at the likelihood of various outcomes.
We start with part terminology.
Events and also Outcomes
The an outcome of one experiment is dubbed an outcome.An event is any certain outcome or group of outcomes.A simple event is an event that cannot be broken down furtherThe sample space is the collection of all feasible simple events.example
If we roll a traditional 6-sided die, describe the sample space and some an easy events.
Show Solution
The sample space is the set of all possible simple events: 1,2,3,4,5,6
Some instances of an easy events:
We role a 1We role a 5Some link events:
We role a number bigger 보다 4We roll an even numberBasic Probability
Given the all outcomes room equally likely, we have the right to compute the probability of an event E utilizing this formula:
examples
If we roll a 6-sided die, calculate
P(rolling a 1)P(rolling a number bigger 보다 4)Show Solution
Recall the the sample room is 1,2,3,4,5,6
There is one outcome corresponding to “rolling a 1,” for this reason the probability isProbabilities are basically fractions, and also can be lessened to lower terms favor fractions.
This video describes this example and the previous one in detail.
Let’s to speak you have actually a bag with 20 cherries, 14 sweet and also 6 sour. If you choose a cherry at random, what is the probability the it will be sweet?
Show Solution
There space 20 possible cherries that can be picked, therefore the variety of possible outcomes is 20. Of these 20 possible outcomes, 14 are favorable (sweet), for this reason the probability the the cherry will certainly be sweet is
Try It
At some random moment, friend look at your clock and also note the minutes reading.
a. What is probability the minutes analysis is 15?
b. What is the probability the minutes analysis is 15 or less?
Cards
A conventional deck of 52 playing cards is composed of four suits (hearts, spades, diamonds and clubs). Spades and also clubs are black if hearts and diamonds room red. Each suit contains 13 cards, every of a different rank: an Ace (which in many games functions as both a short card and a high card), cards numbered 2 with 10, a Jack, a Queen and a King.
example
Compute the probability the randomly drawing one map from a deck and also getting an Ace.
Show Solution
There room 52 cards in the deck and also 4 Aces so
We can also think of probabilities as percents: there is a 7.69% possibility that a randomly selected card will certainly be an Ace.
Notice that the smallest feasible probability is 0 – if there room no outcomes the correspond v the event. The largest feasible probability is 1 – if all possible outcomes correspond v the event.
This video demonstrates both this example and also the vault cherry example on the page.
Certain and Impossible events
An impossible occasion has a probability that 0.A specific event has actually a probability that 1.The probability of any kind of event must beTry It
In the course of this section, if girlfriend compute a probability and also get solution that is an adverse or greater than 1, you have made a mistake and should inspect your work.
Types the Events
Complementary Events
Now permit us study the probability that an event does not happen. Together in the ahead section, take into consideration the instance of roll a six-sided die and first compute the probability of roll a six: the price is P(six) =1/6. Now consider the probability the we do not role a six: there are 5 outcomes that room not a six, so the prize is P(not a six) =
This is no a coincidence. Consider a generic case with n feasible outcomes and an occasion E that coincides to m of this outcomes. Then the remaining n – m outcomes exchange mail to E not happening, thus
Complement of an Event
The complement of an event is the event “E doesn’t happen”
The notationexample
If you pull a random map from a deck of play cards, what is the probability it is no a heart?
Show Solution
There space 13 understanding in the deck, so
The probability of not illustration a love is the complement:
This instance is described in the complying with video.
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Probability of two independent events
example
Suppose we flipped a coin and rolled a die, and wanted to understand the probability of acquiring a head on the coin and also a 6 top top the die.
Show Solution
We might list all possible outcomes: H1,H2,h2,H4,H5,H6,T1,T2,T3,T4,T5,T6.
Notice there space
The prior instance contained two independent events. Gaining a details outcome from rolling a die had actually no influence on the result from flipping the coin.
Independent Events
Events A and B space independent events if the probability of event B developing is the very same whether or not event A occurs.
example
Are these events independent?
A same coin is tossed two times. The two events are (1) first toss is a head and (2) second toss is a head.The two occasions (1) “It will certainly rain tomorrow in Houston” and (2) “It will rain morning in Galveston” (a city near Houston).You attract a card from a deck, then attract a 2nd card without replacing the first.Show Solution
The probability the a head comes up top top the second toss is 1/2 nevertheless of even if it is or not a head came up ~ above the an initial toss, for this reason these occasions are independent.These occasions are not independent because it is much more likely the it will rain in Galveston on job it rains in Houston than on days it does not.The probability that the second card being red counts on even if it is the first card is red or not, therefore these occasions are not independent.
When two occasions are independent, the probability that both arising is the product that the probabilities that the separation, personal, instance events.
P(A and also B) for independent events
If occasions A and B are independent, climate the probability of both A and B occurring is
where P(A and also B) is the probability of occasions A and also B both occurring, P(A) is the probability of occasion A occurring, and also P(B) is the probability of event B occurring
If girlfriend look ago at the coin and die instance from earlier, you deserve to see exactly how the variety of outcomes of the an initial event multiplied by the number of outcomes in the 2nd event multiplied to same the total number of possible outcomes in the merged event.
example
In her drawer you have actually 10 pairs of socks, 6 the which room white, and also 7 tee shirts, 3 of which space white. If friend randomly reach in and also pull out a pair that socks and also a tee shirt, what is the probability both space white?
Show Solution
The probability of picking a white pair that socks is
The probability of choosing a white tee shirt is
The probability that both being white is
Examples of joint probabilities are debated in this video.
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The previous instances looked at the probability the both events occurring. Now we will certainly look at the probability that either occasion occurring.
example
Suppose we flipped a coin and also rolled a die, and wanted to understand the probability of acquiring a head top top the coin or a 6 top top the die.
Show Solution
Here, there room still 12 feasible outcomes: H1,H2,h2,H4,H5,H6,T1,T2,T3,T4,T5,T6
By merely counting, we deserve to see the 7 of the outcomes have a head on the coin or a 6 on the dice or both – we usage or inclusively here (these 7 outcomes are H1, H2, h2, H4, H5, H6, T6), therefore the probability is
As we would certainly expect,
If us subtract the end this dual count, we have the exactly probability:
P(A or B)
The probability of one of two people A or B occurring (or both) is
example
Suppose we draw one card from a standard deck. What is the probability the we obtain a Queen or a King?
Show Solution
There are 4 Queens and 4 queens in the deck, hence 8 outcomes corresponding to a Queen or King the end of 52 possible outcomes. Thus the probability of illustration a Queen or a King is:
Note the in this case, there space no cards that space both a Queen and also a King, so
See more about this example and also the ahead one in the following video.
In the critical example, the occasions were mutually exclusive, so P(A or B) = P(A) + P(B).
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example
Suppose we attract one card from a traditional deck. What is the probability the we acquire a red map or a King?
Show Solution
Half the cards room red, therefore
There are 4 kings, for this reason
There are two red kings, for this reason
We can then calculate
Try It
In her drawer you have actually 10 pairs of socks, 6 of which space white, and also 7 tee shirts, 3 the which are white. If you with in and randomly take a pair of socks and a tee shirt, what the probability at the very least one is white?
Example
The table listed below shows the number of survey topics who have actually received and not got a speeding ticket in the last year, and the color of their car. Uncover the probability the a randomly preferred person:
Has a red car and acquired a speeding ticketHas a red vehicle or acquired a speeding ticket.| Speeding ticket | No speeding ticket | Total | |
| Red car | 15 | 135 | 150 |
| Not red car | 45 | 470 | 515 |
| Total | 60 | 605 | 665 |
Show Solution
We can see that 15 civilization of the 665 surveyed had both a red car and also got a speeding ticket, therefore the probability is
Notice that having actually a red car and also getting a speeding ticket are not live independence events, so the probability of both the them arising is not simply the product that probabilities of every one occurring.
We could answer this question by simply including up the numbers: 15 human being with red cars and also speeding ticket + 135 through red cars however no ticket + 45 with a ticket yet no red vehicle = 195 people. So the probability is
We additionally could have discovered this probability by:
P(had a red car) + P(got a speeding ticket) – P(had a red car and also got a speeding ticket)
=
This table example is detailed in the following explanatory video.
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Conditional Probability
In the previous ar we computed the probabilities of events that were independent of every other. We observed that acquiring a details outcome native rolling a die had no influence on the result from flipping a coin, also though we were computing a probability based upon doing them at the very same time.
In this section, we will consider events that are dependent on every other, dubbed conditional probabilities.
Conditional Probability
The probability the occasion B occurs, given that occasion A has actually happened, is stood for as
P(B | A)
This is check out as “the probability of B offered A”
For example, if you draw a card from a deck, then the sample an are for the following card attracted has changed, due to the fact that you are now working v a deck that 51 cards. In the following instance we will display you how the computations for events like this are different from the computations we did in the critical section.
example
What is the probability that two cards drawn at arbitrarily from a deck of play cards will certainly both be aces?
Show Solution
It can seem that you might use the formula for the probability of two independent events and simply multiply
Once the first card favored is an ace, the probability that the second card favored is also an ace is called the conditional probability of illustration an ace. In this instance the “condition” is that the very first card is an ace. Symbolically, we write this as:
P(ace on second draw | one ace top top the an initial draw).
The upright bar “|” is review as “given,” therefore the above expression is brief for “The probability the an ace is attracted on the 2nd draw given that an ace was attracted on the first draw.” What is this probability? after ~ an ace is drawn on the an initial draw, there space 3 aces out of 51 complete cards left. This means that the conditional probability of drawing an ace after one ace has currently been drawn is
Thus, the probability that both cards gift aces is
Since we understand the person has actually a red car, us are just considering the 150 civilization in the an initial row that the table. The those, 15 have a speeding ticket, for this reason P(ticket | red car) =
Notice native the last instance that P(B | A) is not equal to P(A | B).
These kinds of conditional probabilities room what insurance carriers use to determine your insurance allowance rates. Castle look at the conditional probability the you having actually accident, given your age, your car, your car color, her driving history, etc., and price your policy based on that likelihood.
View more about conditional probability in the complying with video.
Example
If you draw two cards indigenous a deck, what is the probability that you will get the Ace the Diamonds and a black card?
Show Solution
You can satisfy this condition by having case A or case B, as follows:
Case A) girlfriend can obtain the Ace that Diamonds an initial and climate a black card or
Case B) friend can gain a black color card an initial and climate the Ace that Diamonds.
Let’s calculation the probability of situation A. The probability the the an initial card is the Ace that Diamonds is
Now for situation B: the probability the the an initial card is black color is
Recall that the probability of A or B is P(A) + P(B) – P(A and B). In this problem, P(A and also B) = 0 because the very first card cannot be the Ace the Diamonds and be a black color card. Therefore, the probability of instance A or case B is
These two playing card scenarios are discussed further in the complying with video.
See more: Uci Beall Center For Art And Technology At Uc Irvine, Uc Irvine, Beall Center For Art And Technology
Try It
Since we know the test result was positive, we’re limited to the 75 females in the very first column, of i beg your pardon 5 were not pregnant. P(not pregnant | confident test result) =
The second result is what is usually dubbed a false positive: A positive result when the mrs is no actually pregnant.